In some trucks and heavy duty trailers there is a unit of four supporting wheels and tires. The unit comprises walking beams. In such a unit there are two walking beams. Also, there are four bushings in such a unit.
In a tandem axle truck such as a dump truck there are eight wheels in a unit and four bushings in a unit having two walking beams and tandem drive axles. A tandem axle dump truck hauls and conveys a heavy load such as dirt, sand, gravel and rocks. The tandem axle dump truck is used both on the highway and off the highway. Quite often, when the tandem axle dump truck is used off of the highway the terrain is rough, bumpy, and has recesses which are hard on the truck and the walking beam. In these situations the bushings in the walking beam must be replaced at approximately 50,000 miles to 60,000 miles. The bushings are subject to considerable punishment and if the bushings are not replaced the truck does not handle properly and is difficult to manuever and work and can be a hazard to drive. Also, the side walls of the tire may fail.
A tandem axle logging truck is used both on the highway and off of the highway. Some logging trucks will have a walking beam unit comprising four wheels and two bushings. Many times a logging truck travels relatively many miles on the highway and only a relatively few miles off of the highway. In these instances the bushings in the walking beam are not placed under pressure as much as the bushings in a dump truck and it is possible to secure 250,000 miles or maybe 300,000 miles on the bushings in a walking beam in a logging truck without the need for replacement.
Certain trailers may have a walking beam unit comprising four wheels and two bushings. Generally, trailers are used on the highway and since the highway is relatively smooth without recesses or dips the bushing does not take the beating that a bushing will take for an off highway truck such as a dump truck. In these instances it is possible to use bushings in a trailer for 250,000 to 300,000 miles.
In the walking beam there is a shaft and caps and also a rubber bushing surrounding the shaft. There surrounds the rubber bushing a hanger. With the wearing of the rubber bushing it is difficut to separate and to replace the rubber bushing after the walking beam has been used for a considerable number of miles or a considerable period of time. Between the shaft and the cap and also the rubber bushing there is some dirt, grime and some rust. To separate the cap from the shaft is difficult. If the cap can be separated from the shaft then the shaft can be forced out or pressed out of the rubber bushing. This allows the rubber bushing to drop and to be replaced with a new rubber bushing. The separation of the shaft and cap from the rubber bushing is a dirty, messy job. It is necessary to crawl under the truck or under the trailer. One way of separating the cap and the shaft is to use a chisel and to force apart the shaft and the cap. The cap is forced away from the shaft by using a hammer and chisel and manual labor. As there is a set of four wheels in a unit the time required to force apart the caps and shafts and to remove the old rubber bushing and to insert a new rubber bushing, a new shaft and a new cap is approximately 12 to 14 hours. This means that the truck or trailer is out of service for a period of 12 to 14 hours. The value of a truck in service or the value of a trailer in service is placed at about $100.00 per hour. Because the truck or trailer is out of service for 12 to 14 hours there is a loss of about $1200.00 to $1400.00 in the use of the truck or trailer. The loss of $1200.00 to $1400.00 is in addition to the cost of replacing the rubber bushing. The cost of the replacing of the rubber bushing is approximately $900.00.
Instead of using a hammer and a chisel to separate the cap and shaft it is possible to use a cutting torch. The flame of the cutting torch can be adjusted so as to cut through the rubber bushing and the shaft. The rubber bushing is burned away and also the shaft is burned away. Then, a new shaft and a new rubber bushing and new caps can be inserted. A drawback to the use of a burning torch or a cutting torch is the smell of burning rubber. It is necessary to have eight cuts or eight burns with the torch. The time to accomplish eight burns or eight cuts with the torch is approximately one and one-half hours to remove the shafts and rubber bushings. The total time to replace the rubber bushings in a unit of four wheels is approximately three hours. This means that there is a lose of time and a loss of money of about $300.00 in the replacing of the rubber bushings as the truck or trailer cannot be used. The manufacturer advises against the use of a cutting torch.
Another way of removing the rubber bushings, other than by a hammer and a chisel or a cutting torch, is by a saw.
A more complete discussion of walking beams and bushings is presented by Hendrickson Mfg. Co., P. 0. Box 249, Burr Ridge, Ill., 60521, see publications, SR SERIES and RT/RTE SERIES.
A bushing may be of a suitable plastic as well as of rubber.